Packaging laminates are used for enclosing food products, especially liquid food such as drinks and other beverages. A typical laminate consists of a core layer of paper based material being covered on one side by a thermoplastic polymer layer facing the outer environment of the package to be formed. The inside of the packaging laminate may comprise a multi-layer structure, including a first layer of polymer being in contact with the core layer, a barrier film of Al, and subsequent polymer layers to be in contact with the enclosed product.
In certain applications the packaging laminate may be provided with pre-laminated holes, i.e. areas where the core layer is removed such that the outer thermoplastic polymer layer is in direct contact with the inside multi-layer laminate structure. Such pre-laminated holes may e.g. be provided for facilitating the arrangement of opening devices such as re-closeable caps, or as straw holes. By removing the core layer of paper based material at the position of the pre-laminated holes, easy opening of an opening device arranged at the position of the pre-laminated hole is enabled.
Generally, the position of the pre-laminated hole is determined by punching the core layer before lamination.
During the provision of pre-laminated holes different failure modes may occur. For example, air may be trapped between the Al foil and the inside polymer film. The presence of such air inclusions may cause cracks or open holes in the polymeric layers during lamination, and it will thus affect the aseptic performance of the package. Another example is the formation of pinholes, which may occur during extrusion coating of the polymeric layers. Since the aseptic performance of the packaging laminate is critical, it is desired to have a method for detecting such defects.
One such method relies on the operation of a conductivity meter. In this method, the surface of the pre-laminated hole is provided with a thin film of electrolyte, e.g. NH4Cl at a concentration of 20 g per liter water. The Al foil is connected to a low voltage electrode, and a second electrode, having a planar surface of approximately 3 mm in diameter, is moved in close contact over the pre-laminated hole. When applying a low voltage to the electrodes, in combination with a high resistance in series with the test circuit and a signal amplifier, pin holes creating a contact and thus a closed circuit are easily detected.
Another system presently on the market uses a high voltage roller electrode that continuously passes over a grounded substrate. A spark over is created and registered when the electrode passes over a defective spot.
Although this known system may be suitable for some applications, it may not be used in an advantageous way to register defects of a pre-laminated hole, since the topography is disruptive at the edges of such holes. This means that the roller electrode will loose contact with the pre-laminated hole and consequently it will have an inconsistent sensitivity for any defects on this area. Moreover, the known system requires a dry sample since water on the surface will create a short circuit on the sample.